This invention relates to case hardening of metal or alloys and, more particularly, to case hardening with a nitrogen and metal or alloy solid solution.
For components formed of metals or alloys it is often desirable to form a hardened surface case on a core of the metal or alloy to enhance the performance of the component. The hardened surface case provides wear and corrosion resistance while the core provides toughness and impact resistance.
There are various conventional methods for forming a hardened surface case. One such typical method, nitriding, utilizes gas, salt bath, or plasma processing. The nitriding process introduces nitrogen to the metal or alloy surface at an elevated temperature. The nitrogen reacts with the metal or alloy to form hard nitride compounds on the metal or alloy surface. This conventional process provides the benefit of a hardened surface case, however, the nitride compounds may be brittle, friable, cause premature failure, or be otherwise undesirable.
The nitride compounds may include a variety of different compositions, such as the ε and γ′ compositions of iron and nitrogen, as well as various different compositions and crystal structures. The formation of nitride compound compositions introduces some volume fraction within the transformed surface region that possesses properties that are dissimilar to those of the substrate. While the microstructural and compositional transitions are gradual, the presence of nitride compounds having dissimilar properties can lead to deleterious performance in applications that involve contact stress, such as gears and bearings.
Accordingly, it is desirable to provide a method of case hardening that avoids an abrupt change in composition and crystal structure by forming a solid solution region having a gradual transition in nitrogen concentration between the case surface and the core.